The invention relates to the technical field of the production of plasma by electron cyclotron resonance (ECR) from a gaseous medium, and relates more particularly to a surface treatment.
In a manner perfectly known to a person skilled in the art, the plasma generated by electron cyclotron resonance can be used in the surface treatment of metal or other parts, such as the cleaning of the parts by ionic etching, ionic assistance to a PVD deposition process, the activation of gaseous species to make PACVD coatings, etc. These plasma surface treatment methods can be used, inter alia, in the field of mechanical engineering, optics, corrosion protection, or surface treatment for power generation.
The invention finds a particularly advantageous application in the treatment of one or more parts having multiple faces to be treated, or even shapes called complex shapes.
In a manner known per se, to carry out a plasma treatment on parts of complex shape, the said parts are polarized in continuous, pulsed or radiofrequency mode, in order to create a plasma directly at the surface of the parts to be treated. The drawback of this solution is that the production of plasma and the bombardment of the surface by the plasma ions are not independent, because the working voltage determines both the plasma density and the energy of the ions which reach the surface. It is also observed that this polarization is only effective at pressures of about 1 to 10 Pa. In fact, such a pressure, which is relatively high for a plasma treatment, corresponds to a short mean free path, making the transport of material from or to the parts to be treated particularly difficult, and promoting the formation of hollow cathodes between the parts, so that the treatment obtained is not uniform. To avoid the formation of hollow cathodes, it is necessary to closely monitor the distances between the parts to be treated.
Even when the working pressure is lowered by using an external source to create the plasma, for example radiofrequency waves or microwaves, the problem of the uniform treatment of the parts subsists.
Numerous technical solutions have been proposed to create a uniform plasma at the surface of the part(s) to be treated.
According to the teaching of patent FR 2 658 025, the uniform plasma is obtained by applying a uniform magnetic field in volume. The uniform treatment of the part therefore results from the homogeneity of the plasma. Moreover, this type of configuration is extremely limiting because parts of a ferromagnetic type necessarily modify the homogeneity of the magnetic field and, consequently, the uniformity of treatment.
Patent FR 2 838 020 proposes a distribution of sources on the periphery of a reactor to confine the plasma, so that it is uniform in volume, to obtain a uniform treatment. According to this solution, the magnetic or other nature of the parts is immaterial, but the homogeneity of the plasma is necessarily affected by the presence of the parts. In fact, the production of a plasma that is uniform in volume results from the sum of the contributions of the elementary sources distributed on the wall of the treatment system. The fact of placing an object in the plasma necessarily causes a shadowing of the sources, which is detrimental to the homogeneity of the plasma and the uniformity of treatment.
One solution for treating objects having a planar geometry or having a slightly curved surface geometry, appears from the teaching of patent FR 2 797 372. According to the teaching of this patent, the plasma sources are positioned at a constant distance from the surface of the parts to be treated, so that the plasma is uniform on this surface. However, as indicated, this solution is specific to the predetermined geometry of the parts. For each new geometry, it is necessary to modify the reactor and more particularly, the position of the plasma sources.
Another solution for treating objects having a planar geometry appears from the teaching of patent WO2007/023350. This patent uses an unusual magnetic induction resonance condition B=πmf/e where f is the frequency of the electromagnetic wave and me and e are the mass and charge of the electron, respectively. According to the teaching of this patent, a uniform treatment zone is obtained by bringing the elementary sources sufficiently close together to obtain a common induction isosurface B which satisfies the above resonance condition. At the frequency of 2.45 GHz normally used, this condition is satisfied for a field of about 437 gauss. This solution accordingly requires the use of rather short distances between the elementary sources, less than 5 cm regardless of the pressure. It is therefore necessary to have many individual sources, thereby increasing the cost of treatment. Similarly, the magnetic field facing the sources is reinforced, which may be detrimental for treating magnetic parts.
Another solution appears from the teaching of patent FR 2 826 506, which relates to a device for amplifying the current of an abnormal electrical discharge. The device amplifies an existing plasma and therefore, to function, requires a combination with another electrode such as a magnetron cathode. The plasma is amplified by the application of a positive voltage. In fact, a highly positive plasma has the potential drawback of the sputtering of the reactor walls and, consequently, the contamination of the parts to be treated.
It is the object of the invention to remedy these drawbacks simply, safely, effectively and efficiently.
The problem that the invention proposes to solve is to carry out a surface treatment for treating parts of complex shape uniformly, in other words, for a uniform treatment in volume although the plasma itself is not uniform in volume, thereby limiting the number of individual sources and reducing the cost of the treatment.